Polycarbonates are resins with excellent transparency and heat resistance, as well as excellent mechanical strength including impact strength, and they are therefore used as engineering plastics in a numerous and wide variety of fields. In recent years, branched polycarbonates having a branched structure in the molecular chain are increasingly being used for their excellent molding properties for large-sized molded articles including blow molding of containers, and profile extrusion purposes.
Branched polycarbonates are produced by adding polyfunctional compounds such as trifunctional or tetrafunctional hydroxy compounds as branching agents for production of polycarbonates from aromatic dihydroxy compounds and phosgene. In recent years, non-phosgene methods, known as (melt) transesterification methods, have been a focus of interest as methods for producing polycarbonates, but because such production methods involve polymerization reaction at high temperature, they do not easily yield branched polycarbonates exhibiting the required properties.
In production of polycarbonates by transesterification, it is known that Kolbe-Schmitt reaction or reactions via production of isoalkenylphenols occur as side reactions during the polymerization process, forming branched structures in the polycarbonates (Non-patent documents 1 and 2). These side reactions can have major effects for the production of branched polycarbonates with satisfactory properties. Specifically, when a branched structure created by such a side reaction is introduced simultaneously with the branched structure resulting from the polyfunctional compound, it is possible to obtain a polycarbonate having a specific extent of branching, suitable for production of blow molded articles, but this is also accompanied by coloration that impairs the hue. In order to avoid such coloration, it has been attempted to inhibit such side reactions by using specific compounds such as nitrogen-containing basic compounds as catalysts, so that branched structures are introduced only from the added polyfunctional compounds (Patent documents 1 and 2).
As the complete reverse of this concept, there have been proposed methods of producing branched polycarbonates without using polyfunctional compounds as branching agents, and instead actively inducing side reactions to form branched structures (Patent documents 3-8).